Expandable rod system to treat scoliosis and method of using the same

ABSTRACT

Correction of a scoliotic curve in a spine comprises the steps of implanting an expanding rod isolated completely under the skin and attached to selected portions of the scoliotic curve of the spine at opposing ends of the rod; and producing a controlled force by means of expansion of the rod over at an extended time period under external control until a desire spinal curve is obtained. An incremental force is generated to stretch the scoliotic curve of the spine between the selected portions where attachment of the rod is defined. The controlled force is provided steadily for at least one month or alternatively 1-3 months. Multiple rods may be employed each associated with a different scoliotic curve of the spine or a different portion of the scoliotic curve.

RELATED APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of medical devices for treatingscoliosis and methods of treatment using such medical devices, ormethods of using such devices.

2. Description of the Prior Art

Adolescent (also called idiopathic) scoliosis is believed to be thedirect result of a tight ligamentum flavum during rapid growth. Theligamentum flavum is a strong discontinuous (or segmental) ligament fullof elastic fibers (which gives it its yellow color) that runs along theposterior aspect of the spinal canal. The posterior aspect of the dura,at any given spinal level, touches lamina for one third of its length,and ligamentum for the other two thirds. In the lumbar spine it acts asthe center of rotation (and maybe the center of rotation for thethoracic as well, which would help explain the limited rotationavailable in the thoracic spine).

If indeed scoliosis is the result of a tight ligamentum flavum, thencorrection of scoliosis should be achievable by mechanical stretching.Rapid correction of a scoliotic curve is now possible because greatforce through multiple points of solid fixation are now available. Thesemethods still, however, are described as steps toward insertinginstrumentation for a fusion of vertebrae. A slower and steady stretchover time, would be much less violent, and would allow the use of muchless force and fewer points of fixation.

What is needed is some kind of device and mechanics to perform theneeded mechanical stretching.

BRIEF SUMMARY OF THE INVENTION

The illustrated embodiment of the invention includes a method forcorrection of a scoliotic curve in a spine comprising the steps ofimplanting an expanding rod isolated completely under the skin andattached to selected portions of the scoliotic curve of the spine atopposing ends of the rod; and producing a controlled force by means ofexpansion of the rod over at an extended time period under externalcontrol until a desire spinal curve is obtained.

The step of producing a controlled force comprises the step ofgenerating an incremental force to stretch the scoliotic curve of thespine between the selected portions where attachment of the rod isdefined. The step of generating an incremental force to stretch thescoliotic curve of the spine comprises the step of generating a forceusing a curved rod in a direction tending to align with the spine at theselected portions where attachment of the rod is defined.

The step of implanting the expanding rod comprises implanting theexpanding rod posteriorly to the spine.

The step of producing a controlled force by means of expansion of therod over at an extended time period comprises the step of producing thecontrolled force at a predetermined level over time without anycombination with fusion of vertebrae.

The step of producing the controlled force by means of expansion of therod over at an extended time period comprises producing the controlledforce steadily for at least one month or alternatively 1-3 months.

The step of producing the controlled force by means of expansion of therod over at an extended time period comprises the step of producing thecontrolled force steadily for such period of time as is sufficient tostretch the ligamentum flavum of the spine enough to allow the spine tostraighten.

The invention further contemplates repeating the steps of implanting anexpanding rod with multiple rods, where each rod is associated with adifferent scoliotic curve of the spine or a different portion of thescoliotic curve of the spine, and separately producing a controlledforce by means of expansion of each corresponding rod over at acorresponding extended time period.

The step of producing the controlled force by means of expansion of therod comprises the step of producing the controlled force by means of aratcheted longitudinal extension of the rod powered by an energy sourcesbiologically isolated from the rod by a skin barrier.

The invention also comprises an apparatus used for performing each ofthe above embodiments of the illustrated method. The invention thusincludes among other embodiments a kit of a plurality of expanding rodswith multiple rods, each rod being associated with a different scolioticcurve of the spine or a different portion of the scoliotic curve of thespine and separately producing a controlled force by means of expansionof each corresponding rod over at a corresponding extended time period.

The means for producing the controlled force by means of expansion ofthe rod comprises means for producing the controlled force by aratcheted longitudinal extension of the rod powered by an energy sourcesbiologically isolated from the rod by a skin barrier.

Stated in an alternative form, the invention includes embodiments wherethe invention is an apparatus for correction of a scoliotic curve in aspine comprising an expanding rod isolated completely under the skin;attachment screws to couple the rod to the spine at opposing ends of therod; and a source of controlled force coupled to the rod to longitudinalextend the rod over an extended defined period of time under externalcontrol until a desire spinal curve is obtained.

The rod comprises a pair of telescopically coupled sleeves; and means toprevent relative rotation of the sleeves. The apparatus is combined withan external source of power, and in one embodiment the source of thecontrolled force comprises a torsion spring coupled to a second one ofthe pair of sleeves, a torsion motor coupled to the spring fortorsioning the torsion spring, a power pick up electrically coupled tothe torsion motor for powering the motor from the external source ofpower, a ratchet and a screw drive coupled to the ratchet at one end andto a first one of the pair of sleeves at an opposing end, so that aforce applied to the screw drive from the torsion motor to the first oneof the pair of sleeves over time longitudinally expands the collectivelength of the pair of sleeves. In the illustrated embodiment the torsionmotor comprises a muscle wire.

More generally, where the invention is characterized as a combination ofan expanding rod, attachment screws, a source of controlled force, and aratchet, where the source of controlled force produces a force over adefined period of time, which force is continually maintained to somedegree by the ratchet after initial generation by the source until thespine to which the force is steadily applied straightens at least to apartial degree.

While the apparatus and method has or will be described for the sake ofgrammatical fluidity with functional explanations, it is to be expresslyunderstood that the claims, unless expressly formulated under 35 USC112, are not to be construed as necessarily limited in any way by theconstruction of “means” or “steps” limitations, but are to be accordedthe full scope of the meaning and equivalents of the definition providedby the claims under the judicial doctrine of equivalents, and in thecase where the claims are expressly formulated under 35 USC 112 are tobe accorded full statutory equivalents under 35 USC 112. The inventioncan be better visualized by turning now to the following drawingswherein like elements are referenced by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of a telescoping rod of the inventionshowing the telescopic sleeves.

FIG. 2 is a side perspective view of a drive unit of the illustratedembodiment of the invention included in the rod of FIG. 1.

FIG. 2a is a side perspective transparent view of the invention asdepicted in FIGS. 1 and 2 shown in assembled form.

FIG. 2b is a side perspective transparent view of an embodiment of adrive unit having a linear motor.

FIG. 3 is a posterior view, moving from the left to the right of thedrawing, of a straight, lateral bending and scoliotic spinerespectively.

FIG. 4 is a posterior view of the rod of the invention as implanted nextto a bent scoliotic spine in the left most depiction and next to astraightened spine in the right most depiction of the drawing.

FIG. 5 is a side perspective view of a pair of pedicle screws showing arotational coupling to the end of the rod of the invention.

The invention and its various embodiments can now be better understoodby turning to the following detailed description of the preferredembodiments which are presented as illustrated examples of the inventiondefined in the claims. It is expressly understood that the invention asdefined by the claims may be broader than the illustrated embodimentsdescribed below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention comprises a rod, generally denoted by reference numeral10, for the treatment of scoliosis by slow stretching of selectedbiological tissues, for example, a selected portion of the spine, whichis implantable into the body and is under external control. Theinvention allows for surgical treatment of the spine without fusion ofany of the vertebrae, and can be used not only for scoliosis, but alsoin pediatric orthopedics such as for leg or limb lengthening.

For the purposes of the illustrated disclosure, it is assumed thatadolescent (also called idiopathic) scoliosis is the direct result of atight ligamentum flavum during rapid growth. The evidence for thisassumption is impressive and overwhelming. The second assumption made inthe illustrated disclosure for the sake of simplicity is that we aretreating a simple, single curve, although the indications andcomplexities of extending the use of the invention to more complexcurves is explicitly within the scope of the invention. Where multiplerods 10 are implanted each rod 10 may be fixed across a differentscoliotic curve where multiple scoliotic curves exist, or may be fixedacross different portions of a single scoliotic curve. In such a case,each one of the multiple rods can be separately controlled to producethe needed straightening force or expansion.

The proposed device comprises an expanding or telescopic rod 10 as shownin side perspective view in FIG. 1. As described below, the opposingends of rod 10 will be fixed to selected positions on the spine usingconventional surgical screws. The rod 10 must produce a controlledforce, slowly over time, under precise external control, and be isolatedor implanted completely under the skin and protected by the naturalbarrier, which the skin provides. Rod 10 also needs to be small,powerful, simple enough to be readily manufactured, immune to accidentalactivation, and biologically inert. Rod 10 of the illustrated embodimentmeets each and all of these criteria.

The rod 10 can be as small as ½″ (1.27 cm) in diameter and typically canbe made much smaller, subject only to material limitations arising fromthe strength of the rod 10 itself. Rod 10 can be made from anybioengineered or biological inert material desired, and in theillustrated embodiment is fabricated from metal, such as surgicalquality stainless steel. Rod 10 is comprised of an inner hollowcylindrical sleeve 14 telescopically disposed in outer, hollowcylindrical sleeve 16. The outer diameter of cylindrical sleeve 14 ischosen to telescopically slide without binding in the inner diameter ofcylindrical sleeve 16 along their common longitudinal axes. A pin 12 isfixed to cylindrical sleeve 14 and extends radially therefrom through alongitudinal slot 18 defined through the wall of cylindrical sleeve 16to stop rotation of cylindrical sleeve 14 relative to cylindrical sleeve16 while allowing free relative longitudinal movement of cylindricalsleeve 14 relative to cylindrical sleeve 16 at least to the extent ofthe length of slot 18. Sleeves 14 and 16 are shown as circular cylindersin the illustrated embodiment, but pin 12 may be removed andelliptically or other prismatic shapes may be used in place of acylindrical shape, which would equivalently stop or limit their relativerotation to each other. Therefore, when in the specification the term,“sleeve” is used without modification, it should be understood to have ageneral prismatic shape.

The larger cylindrical sleeve 16 is internally threaded. The smalllongitudinally extending post 20 on the right end 22 of the smallercylindrical sleeve 14 as seen in FIG. 1 is where the drive unit,generally denoted by reference numeral 24, attaches as described below.Drive unit 24 as shown in the side perspective view of FIG. 2 is housedor disposed inside the cylindrical sleeve 16 with the left end 40 of atorsion spring 26 disposed within and affixed to sleeve 14 as shown inFIG. 2a . The opposing end 42 of torsion spring 26 is coupled to orbears against a segment end 44 of shell 36. Shell 36 is disposed insleeve 16 as shown in FIG. 2a and rotates within the end of sleeve 14,but is held by a stop(s) 37 from telescopically sliding into sleeve 14.Shell 36 is thus longitudinally fixed relative to sleeve 14, but is freeto rotate relative to sleeve 14 at least through a defined angularrange. Thus, as shell 36 is rotated as described below, torsion spring26 which has on end connected to sleeve 14 and the other end to shell 36will wind up or down depending on the sense of rotation of shell 36.

FIG. 2 shows the drive unit in exploded view which is comprised ofspring 26, motor 28, pickup coil 30, ratchet 32 and screw drive 34.These elements represent the simplest configuration for the drive unit24, although it must be understood that any drive unit now known orlater devised capable of providing the same or similar force as theillustrated drive unit 24 can be equivalently substituted withoutdeparting from the illustrated embodiment of the invention. Because ofenergy transfer considerations relating to coil 30 and wire 38,additional electronic or electrical components may be required asdiscussed below.

The motor 28 comprises a muscle wire 38 wound onto a cylindrical shell36. Muscle wire 38 is well known to the art and is also called nitinolwire. When current is passed through the wire 38, it heats andcontracts. Wire 38 is electrically connected to or in circuit with coil30 in which a current is inductively generated. One end 48 of wire 38 iscoupled or connected to shell 36. The opposing end 46 of wire 38 iscoupled or connected to sleeve 14 at post 20 as shown in FIG. 2a . Thecontraction of wire 38 causes the entire remaining portion of drive unit24 to rotate against the spring 26. In other words, as wire 38 ohmiclyheats due to the current supplied to it from coil 30 and its lengthcontracts, wire 38 will cause shell 36 to rotate. Shell 36 isrotationally coupled or connected to ratchet 32, which rotates withshell 36 in both senses of rotation. Ratchet 32 is coupled to screwdrive 34 through a conventional ratcheting mechanism (not shown) therebycausing screw drive 34 to rotate in one sense of rotation, but slippingwith respect to screw drive 34 in the opposite sense of direction. Thus,the screw drive 34 is rotated within sleeve 16 and driven forward or tothe left in FIG. 1 causing sleeve 14 to telescope out of sleeve 16. Thecombined length of sleeves 14 and 16 thus increases. When the wire 38cools there is no further torque applied to shell 36 by wire 38, andtorsion spring 26 will rotate shell 36 back to its original angularposition within drive unit 24. Grooves are cut into the screw threads toresist rotation of screw drive 34 in the opposite direction therebyinsuring that ratchet 32 slips against the screw drive 34 when shell 3is returned to its original angular orientation. Screw drive 34 remainsin a new advanced longitudinal position within sleeve 16. In this mannerrod 10 can be repeatedly incrementally lengthened by repeated heatcycling of motor 28. Rod 10 can only lengthen and the ratcheting actionof ratchet 32 prevents it from shortening. The ratchet 32 is designed torelease or disengage from screw drive 34 when a screw-driver or Allenwrench is placed into through-hex socket 50 of screw drive 34 duringsurgical placement and adjustment. Torsion spring 26 also provides somelongitudinal compressive force which tends to urge ratchet 32 againstscrew drive 34, but placement of a tool through socket 50 allows ratchet32 to be backed off and to disengage from screw 32. Withdrawal of thetool then allows spring 26 to urge ratchet 32 back into engagement withscrew drive 34. The initial length of rod 10 can thus be manuallyadjusted up or down at the time of its initial surgical implantation.

The pick-up coil 30 receives energy from an external hand-held source ofenergy (not shown), such as a low frequency generator of electromagneticradiation, which is brought into proximity with coil 30. Rod 10 isimplanted beneath the skin barrier, while the source of energy isexterior to the body. The external inductive power source may be drivenat conventional line frequency. In the event that the coil 30 is be ableto efficiently drive the muscle wire 38, then either a storage capacitorwith a control diode can be added in circuit with coil 30, or with morecomplexity, a battery with a diode voltage multiplier, and control diodecould be used. Any means of impedance matching between coil 30 and wire38 on one hand and between coil 30 and the inductive power source on theother may be employed. The use of external power sources and inductivelypowered implanted coils is well known to the art and are routinely used,for example, in charging implanted pacemaker devices.

For a first alternative, energy would be fed into the pick-up coil untilenough was stored in the capacitor to drive the motor 28. Upon thefiring of the motor 28 the hand-held device would sense the discharge,and shut-off for the prescribed lock-out period.

The second alternative is the same, except that an on-board batteryassists in charging the capacitor, and thus requires significantly morecontrol electronics.

Consider now the surgical implantation of rod 10 and its operationwithin the body. Two pedicle screws 52 as shown in FIG. 5 are placed atthe top and two pedicle screws 52 at the bottom of the curve in a spinewhich being treated. Fixed head screws (not shown) are used. Theexpandable rod 10 is run above the fascia and beneath the skin betweenthe two constructs. The rod 10 is then expanded daily typically for 2-3months. The duration of treatment is variable and is determinedaccording to medical considerations, which are not directly relevanthere. After the desired curve in the spine is attained, the screws 52and rod 10 are removed.

FIG. 3 shows moving from left to right in the drawing a posterior viewof a normal straight, normal in lateral bending, and a scoliotic spine.Note that with normal lateral bending as shown in the center there is nosignificant rotation. Indeed, in scoliosis as shown in the right mostdepiction as in lateral bending there is no significant increase inrotation. This is because scoliosis is caused by a tight ligamentumflavum. Because of this scoliosis patients have loss of kyphosis or areeven lordotic, even in the thoracic spine. These facts can make a slightcurve in the expanding rod 10 very useful as shown in the left mostdepiction in FIG. 4. As the spine straightens, rod 10 is also flexibleenough to slowly straighten with the spine as shown in the right mostdepiction in FIG. 4.

FIG. 4 shows the initial rod placement in the left most drawing. As therod 10 expands, the spine has nowhere to go except to have the pediclescrews 52 cut out or for the spine to straighten. Because the expansionis slow, the pedicle screws 52 don't cut out. As the spine straightens,and also because the rod 10 is posterior to the ligamentum flavum and isin a poor position to stretch the anterior longitudinal ligament, thespine will be pushed from a horizontal into a vertical position, thusrestoring normal kyphosis.

In order to allow both rotation, and some play for the re-establishmentof kyphosis, the rod-to-pedicle screw articulation is provided as shownin FIG. 5. Rod 10 is coupled to pedicle screws 52 by means of a pin 54fixed to screws 52. The extend end 58 of pin 54 is tapered and isdisposed into a conical socket 56 defined into the adjacent end 60 ofrod 10. This allows rod 10 to swivel around the pivot provided betweenthe socket-to-pin connection to rod 10 by fixed pin 54 and pediclescrews 52.

Scoliosis can be treated without difficult, conspicuous, (andineffective) bracing; and without fusion. The key is to stretch out theligamentum flavum. The proposed device discussed should be effective instretching the spine out of scoliosis. This device would also haveseveral applications other than the spine.

Many alterations and modifications may be made by those having ordinaryskill in the art without departing from the spirit and scope of theinvention. Therefore, it must be understood that the illustratedembodiment has been set forth only for the purposes of example and thatit should not be taken as limiting the invention as defined by thefollowing invention and its various embodiments.

For example, while a cycling rotational motor 28 with a rotary ratchethas been described, it is entirely within the scope of the inventionthat a cycled linear motor 29 in combination with a linear ratchetingmechanism 31, both shown in FIG. 2b , could be substituted to replacethe rotational motor embodiment illustrated in FIG. 2 a.

Further, while an ohmicly heated motor is the preferred embodiment, itis also possible to conceive of use of a thermally or otherwise poweredpiston motor where the driving fluid may be heated by RF diathermy orultrasound energy.

Still further the motor may be a rotary motor or linear motor with thepropelling element of the motor is magnetically coupled to a movingmagnetic field source and is combined through appropriate gearing to aratcheting mechanism which telescopes rod 10.

Any means by which energy from an external source can be coupled into amotor which will mechanically telescope rod 10 is included within thespirit and scope of the invention.

Therefore, it must be understood that the illustrated embodiment hasbeen set forth only for the purposes of example and that it should notbe taken as limiting the invention as defined by the following claims.For example, notwithstanding the fact that the elements of a claim areset forth below in a certain combination, it must be expresslyunderstood that the invention includes other combinations of fewer, moreor different elements, which are disclosed in above even when notinitially claimed in such combinations. A teaching that two elements arecombined in a claimed combination is further to be understood as alsoallowing for a claimed combination in which the two elements are notcombined with each other, but may be used alone or combined in othercombinations. The excision of any disclosed element of the invention isexplicitly contemplated as within the scope of the invention.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim. Although elements maybe described above as acting in certain combinations and even initiallyclaimed as such, it is to be expressly understood that one or moreelements from a claimed combination can in some cases be excised fromthe combination and that the claimed combination may be directed to asubcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptually equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

I claim:
 1. A non-invasively expandable rod system for treatingscoliosis comprising: a first end configured to be coupled to a firstlocation on a subject's spine; a second end configured to be coupled toa second location on the subject's spine, wherein the second locationand the first location are separated by a first length; a hollow outermember having an axis, a first end, and a second end, wherein the hollowouter member comprises an opening disposed substantially at one or moreof the first end and the second end; an inner member configured to beslideably received by the opening of the hollow outer member andtelescopically slideable within the hollow outer member andsubstantially along the axis; a ratchet contained within the hollowouter member and configured for controlling movement between the innermember and the hollow outer member when the ratchet is in a first,engaged position, the ratchet being adjustable between the first,engaged position and a second, disengaged position, such that thenon-invasively expandable rod system is prevented from shortening whenthe ratchet is in the first, engaged position and is allowed to shortenwhen the ratchet is in the second, disengaged position; and a socket forremovably receiving a tool into the non-invasively expandable rodsystem, the tool configured to adjust the ratchet between the first,engaged position and the second, disengaged position; wherein theexpandable rod system is configured to maintain a subsequent secondlength between the first location on the subject's spine and the secondlocation on the subject's spine, and wherein when in the first, engagedposition, the ratchet contributes to maintaining a position of the innermember with respect to the outer member.
 2. The non-invasivelyexpandable rod system of claim 1, further comprising a drive unitdisposed in the hollow outer member and operatively coupled to the innermember, wherein actuation of the drive unit moves the inner memberlongitudinally relative to the hollow outer member.
 3. Thenon-invasively expandable rod system of claim 2, further comprising ascrew operatively coupled to the drive unit.
 4. The non-invasivelyexpandable rod system of claim 3, wherein the drive unit comprises arotational motor.
 5. The non-invasively expandable rod system of claim3, wherein the drive unit comprises a propelling element magneticallycoupled to a moving magnetic field source.
 6. The non-invasivelyexpandable rod system of claim 2, further comprising an external driveconfigured to non-invasively convey energy to the drive unit, whereinthe conveyed energy actuates the drive unit.
 7. The non-invasivelyexpandable rod system of claim 1, wherein at least one of the first endand second end is configured to be coupled to the subject's spine with apedicle screw.
 8. The non-invasively expandable rod system of claim 7,wherein the at least one of the first end and second end is configuredto have an articulation with respect to the pedicle screw.
 9. Thenon-invasively expandable rod system of claim 1, further comprising ananti-rotation member disposed on at least one of the hollow outer memberand the inner member and configured to prevent relative rotation betweenthe hollow outer member and the inner member.
 10. The non-invasivelyexpandable rod system of claim 1, wherein the ratchet is a rotaryratchet.
 11. A non-invasively expandable rod system for treatingscoliosis comprising: a first end configured to be coupled to a firstlocation on a subject's spine: a second end configured to be coupled toa second location on the subject's spine, wherein the second locationand the first location are separated by a first length; a hollow outermember having an axis, a first end, and a second end, wherein the hollowouter member comprises an opening disposed substantially at one or moreof the first end and the second end; an inner member configured to beslideably received by the opening of the hollow outer member andtelescopically slideable within the hollow outer member andsubstantially along the axis; and a ratchet contained within the hollowouter member and configured for controlling movement between the innermember and the hollow outer member when the ratchet is in a first,engaged position, the ratchet being adjustable between the first,engaged position and a second, disengaged position by the use of aseparate tool which is insertable into the non-invasively expandable rodsystem, such that the non-invasively expandable rod system is preventedfrom shortening when the ratchet is in the first, engaged position andis allowed to shorten when the ratchet is in the second, disengagedposition, wherein the expandable rod system is configured to maintain asubsequent second length between the first location on the subject'sspine and the second location on the subject's spine, and wherein whenin the first, engaged position, the ratchet contributes to maintaining aposition of the inner member with respect to the outer member.
 12. Thenon-invasively expandable rod system of claim 11, further comprising adrive unit disposed in the hollow outer member and operatively coupledto the inner member, wherein actuation of the drive unit moves the innermember longitudinally relative to the hollow outer member.
 13. Thenon-invasively expandable rod system of claim 12, further comprising ascrew operatively coupled to the drive unit.
 14. The non-invasivelyexpandable rod system of claim 13, wherein the drive unit comprises arotational motor.
 15. The non-invasively expandable rod system of claim13, wherein the drive unit comprises a propelling element magneticallycoupled to a moving magnetic field source.
 16. The non-invasivelyexpandable rod system of claim 12, further comprising an external driveconfigured to non-invasively convey energy to the drive unit, whereinthe conveyed energy actuates the drive unit.
 17. The non-invasivelyexpandable rod system of claim 11, wherein at least one of the first endand second end is configured to be coupled to the subject's spine with apedicle screw.
 18. The non-invasively expandable rod system of claim 17,wherein the at least one of the first end and second end is configuredto have an articulation with respect to the pedicle screw.
 19. Thenon-invasively expandable rod system of claim 11, further comprising ananti-rotation member disposed on at least one of the hollow outer memberand the inner member and configured to prevent relative rotation betweenthe hollow outer member and the inner member.
 20. The non-invasivelyexpandable rod system of claim 11, wherein the ratchet is a rotaryratchet.